This invention concerns radar systems and more particularly radar systems adapted to be utilized for anti-collision detection of aircraft, ships at sea and the like.
The constantly increasing density of air traffic in this country and throughout the world threatens to overburden the present system for managing air traffic around high traffic density areas, i.e., large metropolitan area airports. These present-day systems basically consist of an ordering of traffic by air traffic controllers utilizing ground-based radar and radio communication within the range of authority of a particular air control center.
Dangerous conditions potentially may arise from the increased possibility of air traffic controller error and/or errors by the aircraft pilots. In addition, the possibility of collision although somewhat reduced still exists in the air space outside traffic controlled air space.
Under conditions of reduced visibility, dangerous situations arising from such human error or other causes may not be easily avoided due to the relatively high speed at which aircraft are operated, sometimes requiring immediate evasive action to be taken.
Accordingly, anti-collision systems in addition to traffic control have been proposed and devised in the past. Generally, such systems have relied on ground-based radar which monitors the position of aircraft within a given air space and, with the aid of high speed computers, the positional and course data is processed to detect possibly dangerous situations arising from collision courses between detected aircraft.
Such systems of necessity involve enormously complex data processing which taxes even the capabilities of modern day high speed digital computers; furthermore, such systems provide no assistance in areas which are not monitored by the ground-based radar.
Simple anti-collision systems have also been devised in the past in which by the detection of proximity between crafts as by radio transponders, an alarm is set off to alert the pilots.
However, such alarm systems as heretofore devised have not been able to distinguish reliably the truly dangerous situations in crowded areas such as around metropolitan airports. Many false alarms are indicated tending to reduce pilot reliance on the system.
Similar situations sometimes arise in other environments such as with vessels on relatively high density sea lanes in darkness or heavy weather and in remote areas precluding the use of ground-based radar. The radar systems which are available only detect the proximity of a potential hazard rather than the existence of a dangerous collision course between two relatively moving ships.
High frequency (ten gigahertz) radar systems have heretofore been provided which will detect relative velocities or rates of closure between the craft carrying the radar system and other detected craft. Such systems have relied on the "doppler shift" effect on the radar transmission when it returns to the receiver, in order to detect relative rates of velocity between the craft carrying the transmitting radar equipment and the detected object while moving relative to the ground. In such systems, the frequency shift is detected by a narrow band pass radar set receiver. The relative velocities involved for which the system is designed to detect is such that doppler frequency shift is very slight.
The airborne radar set transmits a narrow beam which is sequenced through a spectrum of frequencies which are related to the frequency to which the receiver is tuned such that the entire range of possible relative rates of motion which may be encountered for all speeds of the carrying aircraft will result in a return echo of a frequency falling within the tuning of the receiver. That is, the doppler shifting effect on the transmitting frequency for each frequency increment in the scanned spectrum corresponds to an increment of the possible relative rate of motion between the aircraft and the object moving relative to the ground. If the object is receding, the doppler shift produces a reduction in frequency and if the object is approaching the aircraft, the frequency will be increased, shifting the return signal into the receiver frequency.
An inherent problem with this system involves the confusion of a ground echo with an echo received from a detected moving object. That is, when a return echo is received from the ground, this may tend to obscure the signals returned from the detected object.
The ground echo frequency of course shifts with the direction of the radar transmission with respect to the direction of movement of the aircraft and also varies with the relative ground speed of the aircraft.
The approach heretofore has been generally to merely blank out the ground echo with a signal processing system.
A further design defect in these systems is that the entire range of relative motion which could possibly be encountered by aircraft are scanned regardless of the ground speed of the airborne radar system. This results in the necessity to scan the widest range of possible relative rates of motion even though such rates of motion may be impossible at the ground speed at which the craft is actually moving.
That is, the highest possible rates of relative motion are generally the possible rate of speed of the carrying aircraft added to the possible ground speed of other aircraft which may be encountered. Thus, at relatively low ground speeds of the carrying aircraft, possible rates of motion are much less than that at relatively high speeds. Despite this, current designs scan the entire range of possible rates of relative motion. This requires a longer scan time and greater expenditure of energy than is inherently necessary in order to scan for all possible moving objects.
Accordingly, it is an object of the present invention to provide a radar system particularly suited for anti-collision in which the radar system is carried by the equipped craft and detects movement relative the ground and rates of motion with respect to the carrying aircraft.
It is a further object of the present invention to provide such a radar system in which the transmission of a ground echo producing signal is avoided automatically even though such ground echo transmission frequency varies with ground speed of the carrying vehicle and heading of the radar antenna.
It is still a further object of the present invention to provide such a radar system in which the transmitted sequenced frequencies of the spectrum are related to the ground speed of the carrying craft such as to maximize the efficiency of the scanning for possible detection of objects moving relative to the ground.